For example, Patent Document 1 describes an engine, in which an inside of a combustion chamber is divided into a central combustion chamber and a main combustion chamber by a cavity recessed on a lower surface of a cylinder head and a protrusion part provided with a convexity on a piston crown surface in order to improve a theoretical thermal efficiency of a spark ignition type gasoline engine, and with a compression ratio of the entire combustion chamber set high to the extent of 16 degrees, an air-fuel mixture is set relatively rich inside the central combustion chamber and set relatively lean inside the main combustion chamber so that the combustion chamber of the engine has a lean air-fuel mixture on the whole.
Moreover, for example, Patent Document 2 discloses a technology in which a surface which divides and forms the combustion chambers of an engine is composed of heat insulating materials, including a large number of bubbles, from the point of view of improving the thermal efficiency by reducing the cooling loss. The compression ratio of the engine is 16 in Patent Document 2.